Main Boundary Thrust Zone Research Articles

Delineation of an exhumed intermediate‐depth crustal fault in a collisional setting: An example from the Himalaya

AbstractThe regionally prominent main boundary thrust (MBT) of the Himalayan fold‐thrust belt in northwest India is typically defined by the presence of Proterozoic rocks in the hanging wall and Cenozoic rocks in the footwall. The present study focuses on identifying the MBT contact across Gambar River section in Himachal Pradesh, India, using alternative methodologies, such as the meter‐scale litho‐structural mapping, followed by detrital zircon U–Pb geochronology to precisely identify the thrust contact and provide insights on the deformation history of the MBT zone. We have identified a sharp change in the age (from ~600 to ~61 Ma) of the sedimentary units along a narrow zone in the study area by detrital zircon U–Pb geochronology using LA‐ICP‐MS. The sharp change in the detrital zircon U–Pb age data thus delineate the MBT occurring in the area along a < ~1 m thickness. The lithological assemblage and the age data indicate the unified maximum depositional age from ~700 to ~600 Ma for the hanging wall rocks, which have been equated with the Krol Group of the Lesser Himalayan Sequence (LHS). In comparison, the footwall rocks exhibit the maximum depositional age of ~61 Ma and have been equated with the Cenozoic Subathu Formation of the Sub‐Himalayan Sequence (SHS).

Active tectonics in the Main Boundary Thrust zone, Garhwal Himalaya, as evident from palaeoseismic signatures, morphotectonic features and PSI base ground deformation

In the Garhwal Himalaya, the steeply inclined Main Boundary Thrust (MBT) is characterized by landforms that indicate towards tectonic activity in the recent past. Linear active fault trace, sub‐recent fault scarps, strath terrace, uplifted terraces, deflected drainage, and triangular facet cones are some of the morphotectonic evidences of recent tectonic activity along the MBT. The MBT is defined by active fault trace in the form of linear depression and shows a prominent slope break across it. Thrusting of the Lesser Himalayan bedrocks over the colluvial deposits and back tilting of the colluvial deposits are also observed in the MBT zone. The formation of the sag pond and the deposition of the Quaternary lacustrine sediments followed by the deformation of the lacustrine deposits indicate phases of tectonic activities along the MBT. The later phase of deformation is represented by the folded lacustrine and colluvial deposits, where the amplitude of the fold measures about 3 m‐high and the deformed section measures about 23 m in length. The ongoing deformation patterns of the MBT zone have also been monitored with the help of Synthetic Aperture Radar‐based time series analysis. We used Sentinel‐1C dataset in ascending direction with polarization of the VV+ VH acquired between December 2, 2017 to September 7, 2021 to identify phase changes caused by ongoing active deformation. The Persistent Scatterer Interferometry (PSI) result based on Sentinel‐1A suggests that the Dehradun–Mussoorie and adjoining region has undergone an average deformation of 5.5 to −11 mm/year and the ground displacement of 1.5 to −3.8 cm. The time series analysis reveals that the Mussoorie hills in the MBT zone have higher rate of deformation.

Geomorphic assessment of the factors contributing to the evolution of landforms in Ukhaldhunga area, Kosi River valley, Kumaun Himalaya, Uttarakhand

This paper documents the various factors contributing to the evolution of landforms in the Kosi River valley, Ukhaldhunga area of the Kumaun Himalaya, Uttarakhand. The area falls in the major tectonic boundary between the Lesser Himalaya and the sub-Himalaya. Passing through the Parewa-Ukhaldhunga-Dabara area, this tectonic contact is known as the Main Boundary Thrust (MBT), which is characterized by presence of varied landforms. Study of the development of geomorphology, particularly those evolved in the MBT zone, suggests that the morphological features are intimately associated with the tectonic activities related to the movement of rocks along the MBT and the surface processes such as fluvial and mass movement. The various landforms observed in the MBT zone include strath terraces, fill terraces, paleochannels and V-shaped valley that pertain to fluvial genesis; gravity induced landforms such as landslide debris and colluvial fans; and tectonic landforms such as linear fault traces, swerving/swing of river course and linear depressions. Signature of recent tectonic activity along the MBT is observed between Dabara and Khakrakot. The activity is evident in the form of 8-km-long active fault trace trending NW-SE. Normal fault related landforms are observed in the hanging wall block of the MBT. Neotectonic strath terraces and fill terraces are well developed in the MBT zone. Multiple events of recurrence of landslides are evident from exposed section of landslide debris fans. The bedrocks are highly sheared, suggesting successive tectonic movements along the MBT. As a consequence, tectonic wedges have developed in the bedrocks. These wedges have facilitated for the recurrence of a number of landslides. The anthropogenic activities have also aggravated the slope stability. Truncation of older debris fan surfaces by an erosional/fault scarp is observed and the scarp separates the older fans from the youngest debris fan deposit. The scarp runs parallel to the MBT and Kosi River. Swerving of the Kosi River is as a result of the tectonic forcing along active lineaments/faults. The trend of lineaments of the maximum and high density is NE-SW, which is transverse to the trend of the NW-SE trending Himalayan fold-thrust belt.

Quaternary landform study in Kosi and Dabka river valleys in Kumaun sub‐Himalaya: Implication of reactivation of thrusts

Landforms developed along the Main Boundary Thrust (MBT) and Himalayan Frontal Thrust (HFT) zones in Ramnagar area of Kumaun sub‐Himalaya are studied. The geomorphic analyses suggest their spatial and temporal relationship with the reactivation of the boundary thrusts, faults, and associated later developed transverse faults. Mapping of geomorphic features and structural analysis of the MBT and HFT established an intimate relationship between thrusting of rocks along these major thrusts and evolution of landforms. The reactivations of the MBT and HFT have resulted in recent rejuvenation of landscapes formed in the thrust zones. In the MBT zone, the swerving of the channels, linear development of elongated depressions, sudden topographic breaks, and presence of relatively fresh‐looking escarpments, uplifted present ground surfaces, V‐shaped valleys, and occurrence of linear series of dormant and active landslides are observed. Along the Kosi River section, the vertical‐dipping Neoproterozoic Lesser Himalayan rocks have overridden the recent landslide debris along the subsidiary steep reverse faults associated with the MBT. The HFT is topographically represented by an escarpment trending ESE‐WNW along the Himalayan mountain front. In the HFT zone particularly at Ramnagar area, four levels of terraces are noted, the highest being a strath terrace. The surface trace of the HFT is offset by a transverse N‐S trending Ramnagar Fault, which coincides with the course along which Kosi Rver flows. Uplift of the hanging wall block near the HFT has resulted in subsidence of the present ground surface towards the northeast away from the HFT trace. During this process, the transverse streams have vigorously incised the uplifted surfaces. Rivers and streams realigning with the bedrock structure suggest tectonic control on morphology of drainage patterns. Neotectonic activity along the intra‐Siwalik thrust has offset a terrace surface into two surfaces that are separated by a fault scarp of about 50 m high. The bedrocks in the HFT region are gentle to steep dipping towards NW/NE/ESE. Both normal and reverse faults are recognized in the bedrocks. The general trend of the lineaments, which are concentrated in the thrust zones, is E‐W. Two transverse faults trending nearly N‐S cut and offset the HFT. Tectonic activity during the deposition of the Upper Siwalik rocks is evident from the occurrence of liquefaction features. Soft‐sediment deformation structures are observed in fine‐ to medium‐grained friable sandstones. These include up‐welled structures (with vertical to inclined bedding planes), sand dykes, faults (reverse and normal), folds, pseudo‐nodules, and deformed laminations.

Estimation of deformation temperatures, flow stresses and strain rates from an intra-continental shear zone: The Main Boundary Thrust, NW Himalaya (Uttarakhand, India)

This work generates thermal and tectonic data from the mylonitized rocks of the Main Boundary Thrust (MBT). South and north to this major thrust, the Siwalik range and the Lesser Himalaya, respectively have been receiving a renewed interest amongst the petroleum geoscientists. Understanding temperature, flow stress and strain rates related to a major thrust is crucial in thermal-mechanical modelling of the terrains adjacent to it. Strongly sheared Proterozoic Chandpur phyllites and abundant syntectonic quartz veins occur along the MBT zone (one of the major foreland verging thrusts in the Himalayan orogen) near the Sahenshahi temple at Dehradun, Uttarakhand state, India. Microstructure analyses show intense ductile top-to-S shear in terms of S-C fabric and XRD analyses indicate presence of clays e.g., illite and clinochlore. These clays not only bear the signatures of the intense fluid activity, but also enhance the fault movement by reducing the mechanical strength of the fault zone rocks. Thermometry based on qualitative quartz grain boundary mobility (~300–550 °C) and quantitative Laser Raman Spectroscopy for carbonaceous materials (340–370 °C) reveal the maximum estimates of the deformation temperature and metamorphic temperature, respectively. The higher temperature microstructures can be found in lower temperature domains under the presence of fluids. Hence the quartz thermometry estimates a broader temperature range than the Raman thermometry. Previous works report that, the phyllites present at the base of a thrust sheet, experiences deformation at < 500 °C. The thermometric estimates in this study indicate a higher temperature at the base than that at further north known from previous studies, possibly by shear heating. A flow stress of ~6–49 MPa has been calculated from recrystallised quartz grain piezometry. This broad range of flow stress value indicates temporally variable tectonic stress responsible in the evolution of the MBT. Using these magnitudes, ~10 −15 -10 −16 s −1 of strain rate has been estimated. Such a range can also indicate seismic cycles in the tectonically active Himalaya. • Deformation temperatures, flow stresses and strain rates calculated from a shear zone. • The deduced data will be useful in shear zone kinematics. • The deduced data will help in modeling collision tectonics.

Reply to comments by S. K. Acharyya (2019) on “Stress regime changes in the Main Boundary Thrust zone, Eastern Himalaya, decoded from fault-slip analysis, Patra, A. and Saha, D., Journal of Structural Geology, v.120, p.29–47”

Reply to comments by S. K. Acharyya (2019) on “Stress regime changes in the Main Boundary Thrust zone, Eastern Himalaya, decoded from fault-slip analysis, Patra, A. and Saha, D., Journal of Structural Geology, v.120, p.29–47”

Comments on ‘Stress regime changes in the Main Boundary Thrust zone, Eastern Himalaya, decoded from fault setup analysis’ Journal of structural geology, vol 120 (2019), 29–42, by Abhijit Patra and Dilip Saha

Comments on ‘Stress regime changes in the Main Boundary Thrust zone, Eastern Himalaya, decoded from fault setup analysis’ Journal of structural geology, vol 120 (2019), 29–42, by Abhijit Patra and Dilip Saha

Stress regime changes in the Main Boundary Thrust zone, Eastern Himalaya, decoded from fault-slip analysis

The Main Boundary Thrust (MBT) Zone in the Darjeeling foothills, Eastern Himalaya exhibits northerly dipping fault splays, named here as MBT-1 and MBT-2, with Proterozoic Daling Group in the MBT-1 hanging wall, Upper Paleozoic Gondwana rocks between the MBT-1 and the MBT-2, and Neogene Siwalik Group in the footwall of the MBT-2. On the basis of reduced stress tensors from fault-slip analysis and their grouping relative to different stratigraphic units in the MBT zone that crops out over a strike length of ∼20 km, the following successive stress regimes have been interpreted. The earliest brittle deformation recorded only in the basal part of the Gondwana succession was controlled by a pre-Himalayan NW-SE extensional regime. The Himalayan deformation under sustained N-S compressive regime developed in three stages: (a) thrust displacement along the MBT-1, juxtaposing folded Daling rocks against the Lower Gondwana rocks; (b) tilting and fracturing of Lower Gondwana succession and thrust transport over the Siwalik rocks along the MBT-2; (c) strike-slip regime with NW- and NE-trending faults affecting both the Lower Gondwana and Siwalik strata, developed possibly due to increased tectonic loading. The tensor solutions under N-S compressional regime are consistent with the modern plate convergence, supporting a foreland propagating thrust system in the Eastern Himalaya. In addition, an E-W extensional regime led to apparent orogen-parallel extension.

Geomorphologic study of the valley floor in different tectonic segments along Kosi River valley between South Almora Thrust and Himalayan Frontal Thrust: Kumaun Himalaya, India

In the Kumaun Himalaya, a portion of the Kosi River valley of ~90 km in length is chosen to study the fluvial morphology that provides first‐order information about the dynamic response of bedrock channels to tectonic impulse. The Kosi River flows across/along major tectonic boundaries such as the South Almora Thrust, Ramgarh Thrust, Main Boundary Thrust, and the Himalayan Frontal Thrust, and local transverse and longitudinal faults. Varied fluvial landforms correspond to different tectonic settings, lithologies, bedrock channels, hillslopes, large landslides, terraces, and fans. The longitudinal valleys are also the sites for thick aggradational landforms. Some portion of these valleys fall in the areas of active extensional tectonics and is characterized by one of the widest valley floor sections in the Lesser Himalaya. In contrast, the transverse valley sections are incised by deep‐cut v‐shaped valleys and the narrowest valley section. Swerving of the Kosi River is observed in the Ramgarh Thrust and Amel Fault zones and also in the Main Boundary Thrust zone. Recent tectonic activity is evident from the presence of the faulted Quaternary deposits, linear fault scarps, abandoned channels, incised meandering, and multiple levels of terraces/strath terraces. Field observations and the computed ratio of valley floor width to valley height (Vf) corroborate each other. Valleys developed parallel to the strike of faults and bedrocks have relatively broader valleys with higher Vf values whereas in contrast, the valleys developed across the bedrock strike are narrow with smaller Vf values. The results of computed stream length gradient (SL) and steepness (Ks) indices show considerable correlations between the obtained SL and Ks data and the field evidences; high values of SL and Ks are characterized by the presence of knick points observed at the prominent thrusts and faults.

Landslides and Neotectonic Activities in the Main Boundary Thrust (MBT) Zone: Southeastern Kumaun, Uttarakhand

Abstract Main Boundary Thrust (MBT) zone is constituted of some of the landslide prone areas in southeastern part of Kumaun Sub-Himalaya. Role of landslides as natural hazard and hill slope modifying agent is well documented from various part of Himalayan region and southern hills of Kumaun particularly in the MBT zone, which are susceptible to various type of mass movement. The rocks making up the slopes has been put to a number of brittle deformation phases during the movement along the MBT, and are traversed by number of joint sets. In the open slope these intersecting joint sets forms wedges and are the most favorable site for initiation of rockfalls and other types of landslides. Landslides are taking place primarily due to high angle slopes, formation of structural wedges along the free steep slopes, sheared nature of the rocks due to proximity to the MBT and neotectonic activities along the MBT and other transverse faults. Wedge failure is a common type of landslides in rock slopes characterized by multiple joints and acts as sliding planes for the failed blocks. Field observations and wedge failure analysis indicates most of the landslides taking place in MBT zone of Kumaun Sub-Himalaya are joint controlled. Safety Factor analysis suggests MBT zone of Kumaun Sub-Himalayan region is prone to landslides and related mass movements. This zone is also neotectonically active as indicated by various geomorphic signatures such as structurally controlled drainage pattern, offsetting of fan by MBT and formation of number of small lakes.

Active Faulting and Deformation of Quaternary Landform Sub-Himalaya, India

Landforms developed across terrain defining boundary the Main Boundary Thrust (MBT) have imprints of recent tectonic activity. Depositional landforms such as colluvial fan bear signatures of later phases of tectonic activity in the form of faulting of colluvial fan deposits and development of fault scarps. Tectonic geomorphology applied to the MBT zone suggests recent subsurface activity along the MBT and its splay thrusts. Present day tectonic activity of MBT is indicated by ground creeping, thrusting of Lower Siwalik rocks over recent colluvial fan deposit, aligning of series of lakes along splay faults and laterally along a fault, deflected streams, fault scarps and waterfalls. In the present paper we are addressing the tectonic situation in the foothill region of southeastern Kumaun Sub-Himalaya, India based on detailed field work carried out in the region which brought forward some outstanding morphotectonic evidence of neotectonic activities in the MBT zone.

Microearthquake activity in some parts of the Himalaya and the tectonic model

Microearthquake data from temporary/permanent networks in different parts of the Himalaya shed new light on understanding the earthquake generating processes and their relation to tectonic models of the region. The microearthquake activity in Arunachal Pradesh, northeastern Himalaya, is found to be pronounced at the Main Boundary Thrust (MBT) and to its south; the subcrustal earthquakes (depth 50–80 km) occur much below the plane of detachment of the tectonic models proposed by Seeber et al. (1981) and Ni and Barazangi (1984). The MBT is not the seismogenic fault; the earthquakes are generated by strike–slip movement on deep seated hidden faults, transverse to the MBT. The high seismic activity in the Shillong Plateau, about 200 km south of the MBT in the northeast region, is due to the influence of Himalayan collision tectonics to the north and Burmese arc subduction tectonics to the east. The activity in the Plateau is not directly related to the Himalayan thrust belt or seismic belt. These are mostly crustal earthquakes (depth 10–30 km), and are caused by local active faults/lineaments. In the eastern Himalaya, in the Sikkim and Darjeeling area, the seismic activity is found to be clustered mostly to the north of the MBT. The earthquakes occur at a depth range 0–50 km; the majority of them occur below the detachment plane by thrust-faulting. In the central part, in the Nepal Himalaya, lateral variations of the seismic activity are observed, which represent lateral segmentation of the MBT by transverse tectonic features. In the western Himalaya, however, the tectonic models fit well with the microearthquake data. In the Himachal Pradesh of the western Himalaya, the microearthquakes are mostly recorded in the MBT zone, and the hypocentres (depth 0–20 km) are confined above the plane of detachment or on the Basement Thrust. The earthquakes mostly occur to the south of the Main Central Thrust (MCT), which suggests that the MCT is not seismogenic; it is rather a dormant fault. No single tectonic model explains the Himalayan earthquakes.